Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

The Eastern Pondmussel occurs in sheltered areas of lakes, in slack-water areas of rivers and in canals, where it prefers substrates of fine sand and mud at depths ranging from 0.3 to 4.5 m (Clarke 1981; Strayer and Jirka 1997; Bogan 2002). It is found on substrates composed of over 95% sand in the delta area of Lake St. Clair (Metcalfe-Smith et al. 2004). In rivers, the species is restricted to the lowermost reaches (Strayer 1983 and see Figure 3). van der Schalie (1938) reported the presence of Ligumia nasuta in the mouth of the Huron River as “evidently a Lake Erie intrusion.”

The invasion of the Great Lakes by dreissenid mussels began in 1986 (Hebert et al. 1989) and resulted in the near extirpation of native unionids from Lake Erie, Lake St. Clair and the Detroit and Niagara Rivers by the mid-1990s (Schloesser and Nalepa 1994; Nalepa et al. 1996; Schloesser et al. 2006; Schneider pers. comm. 2002). Only isolated communities with reduced species richness and low abundance still survive in several bays and marshes along the U.S. shore of Lake Erie and in the delta area of Lake St. Clair where zebra mussel densities are low. As 93% of historical records for the Eastern Pondmussel in Canada are from areas now infested with dreissenid mussels, there has been a near total loss of habitat for this species. There is, however, some reason for optimism. Zebra mussel densities declined by 50% between 1994 and 1997 in Lake St. Clair (Nalepa et al. 2001) and by about 3-fold between 1992-94 and 1998 in the Detroit River (Schloesser et al. 2006). Hunter and Simons (2004) reported continuing declines in Lake St. Clair in 2001 and concluded that “If the present biomass levels are sustained or reduced further we would expect the impact of zebra mussels on the Lake St. Clair ecosystem to become considerably moderated.”

Whether or not conditions in Lake St. Clair will improve enough to allow recolonization by unionids is unknown. Unionids and zebra mussels have coexisted in Europe at sites where zebra mussels have been present for decades to millennia, but this may be due to their long shared evolutionary history (Strayer and Malcom 2007). Closer to home, Strayer and co-workers have been studying the interaction between zebra mussels and native bivalves in the Hudson River in eastern New York since zebra mussels first appeared in 1991. Population sizes of three unionids and one sphaeriid declined by 65-100% by 1999, but by 2005 all four species had stabilized or recovered and simple exponential decay models predict that these species may persist at population densities about an order of magnitude below their pre-invasion densities (Strayer and Malcom 2007). It should be noted that this is the only North American study to date that has documented recovery of native bivalves after an extended period of post-invasion decline, and it may not be applicable to Lake St. Clair because the impacts of zebra mussels in the Hudson River were more related to competition for food than to biofouling.

Habitat protection/ownership

To the best of our knowledge, there are only two populations of Ligumia nasuta left in Canada. One population is located in delta area of Lake St. Clair, mainly within the territory of the Walpole Island First Nation. These waters are primarily used for hunting and fishing by the Walpole community and are protected from urban development as well as certain recreational uses (e.g., jet skis are prohibited). Walpole Island contains over 12,000 ha of World Class Wetlands, one of the largest wetland complexes in the Great Lakes Basin (The Nature Conservancy 1995 cited in Bowles 2005), and freshwater mussels occupy the transition zone between these wetlands and the open waters of Lake St. Clair. A second population of L. nasuta was recently discovered in Lyn Creek, a tributary of the upper St. Lawrence River near Brockville, Ontario. Lands adjacent to Lyn Creek are mostly under private ownership. However, there are no bridges or settlements along the stretch of river where live animals and fresh shells were found in 2005-06 and the habitat is relatively undisturbed compared to other areas in eastern Ontario (BMNHC 2006).

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